As an emission type electronic displaying device, an electroluminescence device (hereinafter, referred to as ELD) is known. Elements constituting the ELD include an inorganic electroluminescence element and an organic electroluminescence element (hereinafter referred to also as an organic EL element). Inorganic electroluminescence element has been used for a plane light source, however, a high voltage alternating current has been required to drive the element. An organic EL element has a structure in which a emission layer containing a light emitting compound is arranged between a cathode and an anode, and an electron and a hole were injected into the emission layer and recombined to form an exciton. The element emits light, utilizing light (fluorescent light or phosphorescent light) generated by inactivation of the exciton, and the element can emit light by applying a relatively low voltage of several volts to several tens of volts. The element has a wide viewing angle and a high visuality since the element is of self light emission type. Further, the element is a thin, complete solid element, and therefore, the element is noted from the viewpoint of space saving and portability.
A practical organic EL element to be used in the future is required to emit light of high luminance with a high efficiency at a lower power.
For example, in Japanese Patent No. 3093796, disclosed is an organic EL element exhibiting higher luminance of emitting light with a longer life in which a stilbene derivative, a distyrylarylene derivative or a tristyrylarylene derivative doped with a slight amount of a fluorescent compound is employed.
In Japanese Patent Publication Open to Public Inspection (hereafter referred to as JP-A) No. 63-264692, disclosed is an element which has an organic emission layer containing 8-hydroxyquinoline aluminum complex as a host compound doped with a slight amount of a fluorescent compound. In JP-A No. 3-255190, disclosed is an element which has an organic emission layer containing 8-hydroxyquinoline aluminum complex as a host compound doped with a quinacridone type dye.
When light emitted through excited singlet state is used in the element disclosed in the above Patent documents, the upper limit of the external quantum efficiency (ηext) is considered to be at most 5%, because the generation probability of excited species capable of emitting light is 25%, since the generation ratio of singlet excited species to triplet excited species is 1:3, and further, external light emission efficiency is 20%.
Since an organic EL element, employing phosphorescence through the excited triplet, was reported by Prinston University (see M. A. Baldo et al., nature, 395, 151-154(1998)), studies on materials emitting phosphorescence at room temperature have been actively carried.
Such an examples include those reported in M. A. Baldo et al., nature, 403(17), 750-753(2000) and disclosed in U.S. Pat. No. 6,097,147.
As the upper limit of the internal quantum efficiency of the excited triplet is 100%, the light emission efficiency of the exited triplet is theoretically four times that of the excited singlet. Accordingly, light emission employing the excited triplet exhibits almost the same performance as a cold cathode tube, and can be applied to an illumination device.
For example, many kinds of heavy metal complexes such as iridium complexes have been synthesized and studied, for example reported in S. Lamansky et al., J. Am. Chem. Soc., 123, 4304 (2001).
An example employing tris(2-phenylpyridine)iridium as a dopant has been studied in the abovementioned M. A. Baldo et al., nature, 403(17), 750-753(2000).
As other examples, M. E. Tompson et al. have reported, in The 10th International Workshop on Inorganic and Organic Electroluminescence (EL '00, Hamamatsu), a dopant L2Ir (acac) such as (ppy)2Ir(acac), and Moon-Jae Youn.0g, Tetsuo Tsutsui et al., have reported results of an examination using, for example, tris(2-(p-tolyl)pyridine)iridium (Ir(ptpy)3) or tris(benzo[h]quinoline)iridium (Ir(bzq)3) as a dopant, also in The 10th International Workshop on Inorganic and Organic Electroluminescence (EL '00, Hamamatsu).
An example of preparing an element using varieties of iridium complexes has also been reported in abovementioned S. Lamansky et al., J. Am. Chem. Soc., 123, 4304 (2001).
A hole transport material has been used as a host of a phosphorescent compound in order to increase emission efficiency as has been reported by Ikai et al. in The 10th International Workshop on Inorganic and Organic Electroluminescence (EL '00, Hamamatsu). M. E. Thompson et al. have used varieties of electron transport materials as a host material of a phosphorescent compound and have doped a novel iridium complex into the host materials.
An ortho-metalated complex having platinum as a center metal instead of iridium has also attracted attention. Many examples of such complexes having a characteristic ligand have been known (for example, refer to Patent Documents 1-5 and Non-Patent Document 1).
In each case, the luminance and the emission efficiency have been notably improved since the emission is originated from a phosphorescent emission. However, there have been a problem that the emission life has been shorter.
On the other hand, an example has been known in which an emission of white light is obtained by using coexisting monomer emission and excimer emission (refer to Non-Patent Document 2).
However, there has been a problem in the above example that the emission life has been shorter than an organic EL element in which emission of white light is obtained by using a plurality of emission dopants, since the life of an excimer emission has not been long. In addition to that, the emission efficiency of the excimer emission has been below the practical level.
Patent documents 1JP-A No. 2002-332291Patent documents 2JP-A No. 2002-332292Patent documents 3JP-A No. 2002-338588Patent documents 4JP-A No. 2002-226495Patent documents 5JP-A No. 2002-234894Nonpatent literature 1Inorganic Chemistry, 41(12), 3055-3066 (2002)Nonpatent literature 2Advanced Materials., 14, 1032 (2002)